1. Field of the Invention
The present invention relates to an electrophotographic lithograph printing plate material. More particularly, the present invention relates to an electrophotographic lithograph printing plate material capable of forming thereon electrophotographic images resistive to fogging with toner and having a high dimensional stability and an excellent printing durability.
2. Description of the Related Art
Due to recent progress and development of small-size printing machines and automatic printing mechanisms, the emphasis on light printing process is now shifting to the offset printing process.
There has been much research and development going on regarding plate materials for the offset printing process, and as a result, various improved printing plate materials are now being practically used.
Among the conventional offset printing plate materials, an electrophotographic lithograph printing plate material having a photosensitive electrophotographic layer in which electroconductive zinc oxide particles are dispersed as a principal photoconductive material in a binder matrix, is most widely utilized in the light printing industry, because it is cheap and because the process for making the printing plate from the printing plate material is simple and easy.
In a conventional process for producing an electrophotographic lithograph printing plate, the printing plate material is subjected to a corona charging step, an image-forming light-exposing step, a developing step and a fixing step by using a printing plate-making machine to form visible images in the desired pattern on the photoconductive layer.
The developing step can be effected by either a dry developing method in which a mixture of a toner and a carrier consisting of an iron powder is used as a dry developing agent, or a wet developing method in which a developing liquid containing a toner dispersed in an organic solvent, for example, a petroleum solvent, with a high boiling temperature, is used.
The wet developing method for the preparation of the lithograph printing plate is advantageous in that the reproducibility of half-tone images is high, the resolving power is excellent, the plate-making time is short, and no correction is necessary for the formed images. Therefore, the wet developing method is widely utilized for the production of electrophotographic lithograph printing plates.
Due to the recent significant spread of computer systems in the printing industry, the printing plate-making process is now shifting from the above-mentioned analog process to a digital process. Namely, recent electrophotographic lithograph printing plate materials have a photoconductive layer containing a laser sensitizing agent consisting of a cyanine dye capable of sensitizing the photoconductive layer at a wavelength of about 780 nm. For this type of electrophotographic lithograph printing plate material, a computer-to-plate type printing plate-making method is advantageously utilized. In this method, the data in the computer is directly applied to the electrophotographic material by using a semiconductor laser light.
Generally, the electrophotographic lithograph printing material is required to satisfy various properties including image properties, which are common requirements for all the electrophotographic materials, for example, high image color density, anti-fogging property, sharpness, uniformity and resistance to stain on non-imaged portions; and printing plate properties, which are commonly required for all lithograph printing plates, for example, a property that after a developing step is applied, non-imaged portions of the developed lithograph plate can be desensitized to printing ink, a property that the non-imaged portions of the developed printing plate can be made hydrophilic, and a property that the developed printing plate exhibits a high water resistance to a large amount of wetting water applied to the printing plate surface during printing.
To enhance the toner-fogging resistance of images and the water resistance, an attempt has been made to arrange an intermediate layer between a substrate and a photoconductive layer.
For example, Japanese Unexamined Patent Publication (Kokai) No. 58-124,695 discloses an intermediate layer in which the water resistance thereof is enhanced by using an emulsion resin as a binder resin, and the anti-fogging property thereof is enhanced by adding carbon black thereto. However, the carbon black, which is hydrophobic, needs to be dispersed by using a large amount of a surface active agent which absorbs a large amount of water during the developing step, and thus the resultant printing plate has a low water-resistance and exhibits a poor dimensional stability.
Also, Japanese Unexamined Patent Publication (Kokai) No. 56-24,361 discloses an intermediate layer containing electroconductive zinc oxide which is used to enhance the anti-fogging property. In this attempt, to form images free from toner-fogging, the electroconductive zinc oxide must be contained in a large amount in the intermediate layer so as to cause the particles of the electroconductive zinc oxide to come into contact with each other. Therefore, the resultant intermediate layer becomes porous and exhibits a reduced water resistance. Thus, the resultant printing plate material exhibits an unsatisfactory dimensional stability.